1. Field of the Invention
The invention relates to pressure swing adsorption gas separation operations. More particularly, it relates to an improved flow distribution system for pressure swing adsorption vessels.
2. Description of the Prior Art
Adsorption processes have been widely used for the separation and purification of gasses. In recent years, pressure swing adsorption (PSA) systems have been developed for enhanced gas separation operations, particularly for the commercial production of oxygen and/or nitrogen from air. In the operation of PSA systems, an adsorption-desorption-repressurization processing sequence is employed, with the passage of air or other feed gas mixture at an upper adsorption pressure to an adsorption system for the selective adsorption of a more readily adsorbable component of air or other feed gas mixture by one or more adsorbent beds, and the passage through said bed(s) of a less readily adsorbable component. The bed(s) is then depressurized to a lower desorption pressure for the desorption of the more readily adsorbable component from the adsorbent bed(s), and the bed(s) is then repressurized to the upper adsorption pressure as cyclic operations are continued.
In order to fully utilize the adsorbent material employed, PSA systems require uniform flow of gas across the adsorbent bed(s) throughout the PSA processing cycle. In addition, large void volumes and pressure drops in the PSA vessel entrance and exit regions, which have adverse effects on the process performance of a PSA system, need to be mitigated in practical commercial operations. In this regard, those skilled in the art will appreciate that, in PSA systems, e.g. vacuum pressure swing adsorption (VPSA) systems, the adsorbent bed support and the flow distribution system are costly, and become more costly as the diameter of an adsorbent bed increases. The typical bed support system is also prone to adsorbent leakage if not assembled correctly. Repair of a leaking bed support system can be costly and time-consuming. Common bed designs and flow distribution systems that employ bed support plates and associated flow distribution systems, incorporating support ribs, tend to give rise to localized non-uniform gas flow, which results in an undesired penalty in gas separation performance.
Four general approaches have been employed in commercial practice in attempting to achieve uniform flow in PSA vessels. In one approach, described in "How to Design Fluid - Flow Distributors" by D. R. Richardson, Chemical Engineering, May 1, 1961, a pressure drop is added in the form of a perforated plate or screens positioned across the adsorbent bed. This approach is the least desirable for PSA systems because PSA vessels are large and the inlet, pipe velocities are relatively high, resulting in the need for a high pressure drop across the bed to achieve good gas flow distribution. Such a pressure drop, however, increases the PSA system power requirements, which renders the PSA system, particularly a VPSA system, less competitive in satisfying the requirements of commercial operations.
A second approach incorporates a turning plate that is placed from 1 to 3 pipe diameters from the gas inlet pipe of the system. This plate turns the main inlet(s) gas flow and causes it to diffuse into the head area of the vessel rather than impinge directly into the adsorbent bed. This results in a low pressure drop across the adsorbent bed, but with relatively poor flow distribution, because of the vortices set up in the head area of the vessel which, in turn, impinge on the bed.
A third approach relates to the positioning of a flow distribution plate parallel to the adsorbent bed across the entire head region of the PSA vessel. Such a flow distribution plate contains perforated sections of different open areas that force the gas flow reaching the adsorbent bed to be generally uniform. This approach is further described in the Nowobilski patent, U.S. Pat. No. 5,298,226, issued Mar. 29, 1994. This third approach, as with the first two approaches, employs, in practice, support ribs to stiffen the perforated plate in the radial and circumferential directions. Such support ribs cause areas of high and uneven gas flows due to their inherent channeling of the gas flow.
The fourth approach attempting to achieve uniform gas flow incorporates both flow distribution and bed support through the use of graded balls or cylinders that are built up in decreasing sizes to support the bed of adsorbent material in the PSA vessel. A small basket or plate that may not be of the full bed diameter forms an inlet plenum for the PSA vessel. If the bed adsorbent material is approximately 1/16" diameter, for example, the bed support would be multiple 3" deep layers of 1/8", 1/4", 1/2", 3/4", 1" and 2" diameter ceramic balls, e.g. as shown in Norton, Denstone Inert Catalyst Bed Supports Catalog No. 410027/992. The size of the bed support is such that the smaller particles do not fit between the interstricial spaces of the next size larger particles. The difficulty encountered with this approach is that there is no mechanism to control the flow distribution to the adsorbent bed except by lengthening the bed support section. This, in turn, results in more support material costs, greater void volume in the inlet head and higher pressure drops.
It is an object of the invention to provide an improved adsorbent vessel head section capable of achieving a uniform flow of gas to an adsorbent bed therein.
It is another object of the invention to provide an improved PSA vessel capable of achieving uniform gas flow in an adsorbent bed with reduced void volume and pressure drop.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.